The advantages associated with low friction bearings are well known to a multitude of varied industries. High-speed applications with DN (dynamic number) values over 1,000,000 are common place for turbomachinery. These high-speed applications, owing to the fact that rotor imbalance force increases as a square function of rotor speed, require damping. Without damping, transmitted forces through the system would cause many well-known problems such as noise, fretting, loosening of joints, and overall reduced service life. Further, the bearings themselves would have unacceptable life. For these reasons, turbomachinery bearings are not hard mounted within their housings. The skilled rotordynamics design engineer spends the majority of his/her life managing these forces, especially those forces encountered as the rotor goes through its natural frequencies, commonly referred to as “critical speeds”.
Most turbochargers that employ a low friction rolling element bearing use two angular contact ball bearings, with each accepting the thrust load in a given axial direction, that are joined together in what is commonly referred to as a “cartridge”. In a cylindrical coordinate system a bearing may be defined with respect to axial, radial and azimuthal dimensions. Within a bearing housing, referred to as housing in subsequent text, a cartridge is located axially and azimuthally via one or more mechanisms.
For proper functioning, some movement can occur in a radial direction along a radial line typically defined by an azimuthal locating mechanism. The housing usually allows lubricant to flow to an outer surface of the bearing cartridge whereby lubricant can enter the bearing cartridge. Lubricant can also form a film between the housing and the outer surface of the bearing cartridge, which is often referred to as a “squeeze film”. A clearance between the housing and a portion of the bearing cartridge outer surface typically defines a film thickness. From machine to machine, the film thickness varies in thickness and length depending on many design factors such as the viscosity of the lubricant, the specific rotor size and associated imbalance forces, as well as space envelope constraints.
While operational conditions may cause some slight variations in film thickness (e.g., due to radial movement, etc.), the optimal film thickness is usually a design parameter specified by an outer diameter of the bearing cartridge and an inner diameter of the housing. If the clearance is too small, the squeeze film is overly stiff allowing unacceptable rotor imbalance forces to be transmitted from the bearing cartridge to the housing and surrounding system. On the other hand, if the clearance is too large, then the rotor and bearing cartridge has excessive radial freedom from the center axis of the housing. This excessive radial freedom in turn forces excessive clearances necessary to avoid contacts (rubs) between the rotating turbine and compressor wheels and their respective, stationary housings. These wheel to housing clearances are very undesirable as they cause turbulent, secondary air flows which show up as reduced thermodynamic efficiency of the compressor and turbine stages. Thus, the design engineer is forced into a compromise between desired optimal squeeze film damping and unwanted excessive rotor radial freedom.
Overall, an industry need exists for rolling element bearings and/or housings that utilize a squeeze film damper to be optimized for both damping and rotor radial freedom. Various exemplary bearing cartridges and housings presented herein address such issues and optionally other issues.